It should be taped with two layers of mud, one under one on. However in
a garage they generally don't bother with any nice finish.
You can scrap down the existing high spots with a drywall knife and add
another coat. Continue using wider knives and the same procedure until it
gets to the quality you want. Then sand lightly. It might be good practice
for any other work you may want to do later.
The usual error new people make is using too much "mud" and trying to
use a three inch knife to do the whole job.
Drywall is forgiving, in that you can keep working at it until you
figure it out.
Joseph E. Meehan
26 + 6 = 1 It's Irish Math
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